An automatic analyzer, which irradiates a reaction solution obtained by mixing a sample with a reagent, with a light beam from a light source, calculates absorbance from a change in the amount of a transmitted light beam with a specific wavelength, and thus quantitatively determines the concentration of a substance to be measured in the sample in accordance with the Lambert-Beer's law, is widely used. In such a device, a number of cells each holding a reaction solution are arranged on the circumference of a cell disc that repeatedly rotates and stops, and time-series data on the amount of light beams that have passed through the reaction solution in the cells is measured as reaction process data for about 10 minutes at intervals of about 15 seconds by a transmitted-light measuring unit located at a predetermined position while the cell disc rotates, so that absorbance is calculated from a change in the amount of the light beams, and the concentration of a substance to be measured is thus quantitatively determined.
Reactions that are measured with an automatic analyzer include two types: a color reaction of a substrate and an enzyme, and an immune reaction of an antigen and an antibody. An analysis that uses a color reaction is referred to as a biochemical analysis. Examples of test items of the biochemical analysis include LDH (lactate dehydrogenase), ALP (alkaline phosphatase), and AST (aspartate-oxoglutarate aminotransferase). An analysis that uses an immune reaction is referred to as an immune assay. Examples of test items of the immune assay include CRP (C-reactive protein), IgG (immunoglobulin), and RF (rheumatoid factor). Substances that are measured in an immune assay include test items that should be quantitatively determined in a low-concentration region in which the blood level is low. For such items, a latex immunoassay that uses as sensitizers latex particles, which have antibodies sensitized on (bound to) the surfaces thereof, is used. In the latex immunoassay, antigens, which are substances to be measured, in a sample are recognized by antibodies on the surfaces of latex particles contained in a reagent, and thus are bound thereto, so that the latex particles aggregate via the antigens, thus forming aggregates of latex particles. In the conventional automatic analyzer, a reaction solution in which such aggregates are dispersed is irradiated with light beams, and the amount of transmitted light beams that have passed through the aggregates of the latex particles without being scattered is measured. As the concentration of antigens is higher, the sizes of aggregates that are formed after a given period of time has elapsed becomes larger. Thus, the amount of scattered light beams becomes larger, while the amount of transmitted light beams becomes smaller. Therefore, the concentration of antigens can be quantitatively determined from the amount of light beams measured as the reaction process data.
In recent years, a further increase in sensitivity has been desired for latex immunoassays, and attempts have been made to measure not transmitted light beams but scattered light beams. For example, Patent Literature 1 discloses a system for separating transmitted light beams from scattered light beams using a diaphragm and measuring the transmitted light beams and the scattered light beams at the same time. Patent Literature 2 discloses a system for automatically calculating the difference between the intensities of detected light beams with two different wavelengths, and thus determining the analysis results. Other than automatic analyzers, Patent Literature 3 discloses a system for measuring exogenous macromolecules in the blood by determining the difference between a detection signal including a transmitted light beam and a scattered light beam at a wavelength of 670 nm and that at a wavelength of 940 nm.